In-Situ Vp/Vs Reveals Fault-Zone Material Variation at the Westernmost Gofar Transform Fault, East Pacific Rise

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi-periodically hosts ∼M6 earthquakes every ∼5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we use differential travel times between nearby earthquakes to estimate the in-situ Vp/Vs of the fault-zone materials. We apply this technique to the dataset collected by an ocean-bottom-seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs of 1.61–1.69 in the down-dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs may be caused by both pore fluids and chemical alteration. We also find a 5%–10% increase in Vp/Vs in the barrier zone during the 9 months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone.